Last year, Bojan wrote a short diary about how data exfiltration could use Google Chrome extension syncing. The advantage of such a scheme would be pretty obvious: Google Chrome is "normal" software, and traffic to the Chrome Web Store is also expected. The use of Chrome's cloud infrastructure makes detection of such an exfiltration very difficult.

SANS.edu graduate student David Prefer took a closer look as part of a more systematic research project investigating this covert channel. He looked particularly at bookmarks instead of extensions, as extensions may be restricted, and bookmarks use a similar synchronization channel.

David went as far as to develop a simple and effective tool, "Brugglemark," to set up this covert channel. The tool will take any file, encode it as Google Chrome bookmarks, and reconstitute the file at the receiving end. The sync process is fast and not throttled until thousands of bookmarks are synced. More than 200,000 bookmarks may be used with many kBytes each. This allows for a reasonably efficient covert channel, all wrapped nicely in the Google Chrome protocol to sync data and relay by Google's servers.

Google Chrome's "chrome::/sync-internals/" or "about:sync" features are useful to monitor the synchronization progress. It summarizes various diagnostic parameters to identify the status of the sync process and any delays that may occur. 

David's paper can be found here: https://www.sans.edu/cyber-research/bookmark-bruggling-novel-data-exfiltration-with-brugglemark/.

The GitHub repository with his tools can be found here: https://github.com/davidprefer/Brugglemark/.

The same technique should work just as well with other browsers, not just with Chrome. 

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Johannes B. Ullrich, Ph.D. , Dean of Research, SANS.edu
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